The development of microvascular occlusions in individuals with sickle cell anemia (SCA) is only poorly understood. Our recent investigations have revealed a previously unknown abnormality of the sickle erythrocyte: Their abnormal adherence to endothelium, which appears to be due to an aberrancy of RBC membrane surface charge topography. Moreover, preliminary studies suggest that this phenomenon may be of pathogenetic importance in SCA, as evidenced by an apparent correlation between frequency of microvascular occlusions and adherence of sickle RBC to endothelium. We will examine this abnormal cell/cell interaction by continuing to employ monolayers of cultured human endothelial cells which are exposed to RBC suspensions. The adherence of normal and sickle RBC will be examined after various RBC manipulations directed at elucidating the mechanism responsible for aberrancy of surface charge topography. Extensive manipulations of the assay system will be used to examine the nature of RBC/endothelial interactions and to evaluate the role played by factors in the RBC's environment. Other studies will explore the relationship between RBC adherence to endothelium and clinical severity of SCA. Manipulations which enhance or ameliorate in vitro RBC adhesiveness may generate insights of potential therapeutic importance. In the final stages of this proposal, such insights will be applied to an in vivo model which allows direct observation of the intravascular behavior of sickle RBC.